The Shorts 360-300 ticks just about every one of your boxes, except it's no longer in production as far as I know.
If you're a start-up without bottomless pockets, a search for good 360's could pay off well. Reliable, cheap, pax-friendly, and - I would imagine - there's still a qualified pilot and engineer pool. I'll take Capt Chambo's word that it satisfies your performance needs; I haven't done the sums.
I would advise avoiding old and small Dash 8s like the plague. Could be a maintenance and parts nightmare.